The head and several leaders of one of Syria`s most powerful rebel groups, Jaysh al-Islam, have been killed in an air strike east of Damascus, reports BBC.Founder Zahroun Alloush, 44, was among those killed when rockets hit a meeting place, rebels and the Syrian army said.The Saudi-backed Islamist group is one of the biggest factions and is dominant in the Eastern Ghouta countryside.It recently joined an opposition summit in Riyadh which produced a framework for peace talks with the government.Ten rockets struck as Jaysh al-Islam commanders met, Saudi-funded al-Arabiya TV reported. The group`s deputy leader was also killed, al-Arabiya said.